Safety relief valves are commonly used to regulate the pressure in vessels. In a typical installation, a main safety relief valve is mounted on a tank or other pressure vessel for relieving fluid from the vessel if fluid pressure rises above a predetermined maximum safe value. A suitable safety relief valve includes a valve member reciprocal within a valve body. The valve member is normally in a closed position, and moves to an open position to relieve pressure in the vessel. A dome chamber provided in the relief valve is normally at the same pressure as the inlet pressure to the relief valve, and pressure in the dome chamber acts on the valve member to maintain the relief valve closed. A decrease in dome pressure causes the inlet pressure to open the relief valve and relieve pressure from the vessel. U.S. Pat. No. 4,870,989 discloses a safety relief valve, and an improved seal for safety relief valve is disclosed in U.S. Pat. No. 5,011,116. A suitable safety relief valve according to this invention is manufactured by Anderson Greenwood & Co. as a Type 727.
Pilot valves have been used to control the operation of the main safety relief valve, and are preferred over conventional spring valves for many applications. Early versions of pilot valves for controlling a main pressure relief valve are disclosed in U.S. Pat. Nos. 3,512,560 and 3,864,362. U.S. Pat. No. 4,172,466 discloses a pressure responsive valve with a tandem pilot and stabilizing valve assembly mounted on a safety relief valve. U.S. Pat. No. 4,672,995 discloses redundant pilot valves and a control system which allows each pilot valve to be independently actuated for triggering the safety relief valve to open and release excess pressure. The redundant pilot valves are mounted on a manifold body which in turn is supported on the housing of the main relief valve.
In many operations, a block valve cannot be used between the pressurized vessel and either the safety relief valve or the pilot valve, since the inadvertent closure of the block valve would obviate the entire safety control system. A valve selector manifold which allows one safety relief valve and its associated pilot valve to remain in service while a second relief valve and its associated pilot are removed from service is disclosed in U.S. Pat. No. 4,821,772. The solution proposed by the '772 patent allows for service while desirably reducing the number of openings to the pressure vessel, although this solution is relatively expensive since two separate relief valves are utilized.
One of the problems which has long plagued the use of pilot valves to control a main safety relief valve involves the pilot valve maintenance. The pressure in the vessel is frequently maintained near its maximum allowable value, so that pressure to the pilot valve is only slightly less than that required to trigger operation of the main relief valve. Accordingly, the valve element in the pilot valve is not held tightly into engagement with the seat, and instead may "flutter" against the seat without opening sufficiently to activate the relief valve. This causes high wear on the pilot valve, and also allows debris in the flow line to the pilot valve to prevent reliable seating between the pilot valve element and the seat. Accordingly, two pilot valves and a three-way control valve have been hydraulically interconnected by suitable flow lines, so that one pilot valve may be taken out of service during a maintenance operation while the other pilot valve reliably controls the operation of the relief valve. This technique allows for servicing of each pilot valve without shutting down the system protected by the main relief valve.
A prior art safety relief system employed by Anderson Greenwood comprises a pair of pilot valves connected hydraulically in parallel. Pairs of hand valves are connected to a manifold block, and each of the inlet control valves is spaced upstream from the respective pilot valve. A slidable link is movable with respect to the manifold block, and may be positioned so that one pair of hand valves remain open (and are prevented from closing by a mechanical stop) in order to supply vessel pressure to one of the pilot valves and then to the dome chamber of the relief valve. When the link is positioned so that the stop is removed to allow one pair of hand valves to close, the other pair of hand valves was inherently prevented from closing. This feature thus ensured that the vessel pressure would always be supplied to one of the pilot valves while the other pilot valve could be isolated from the system and removed during a pilot valve service operation. A test valve separate from the pair of hand valves allowed for testing of each pilot valve.
While the above solution is considerably less expensive than that provided by the '772 patent, the cost of the hand valves and the separate pilot valves, the mechanism for mounting the pilot valves, the separate test valves, the fittings required to fluidly connect these valves, and the time and expertise required to properly install the tubing which interconnects these valves are a considerable expense to the overall safety relief system. While these costs are justified for many applications in order to obtain the benefits of a pilot operated safety relief system, a standard spring biased relief valve system is used in many applications to reduce costs. Also, many of the flow lines which interconnect the various valves in the pilot operated safety relief system are continually supplied with high vessel pressure. Numerous threaded connections between these flow lines and the valves increase the likelihood of leakage and the resultant release of fluid to be protected by the safety relief system. If a leak developed in a flow line upstream from the shutoff valve to the pilot valve, it may be impossible to maintain the vessel pressure while repairing the leak. In some cases, a flow line leak may result in the premature or faulty operation of the relief valve. Also, the flow lines and the various valves in the pilot operated safety relief system are exposed and thus could be inadvertently ruptured, thereby again requiring a shutdown of the system which pressurized the vessel.
A related problem concerns the time and expense required to replace or repair a pilot valve in a safety system as described above. While one of the pilot valves remained in service and pressure was cutoff to the other pilot valve, the other pilot valve member could be removed from the system, but the pilot valve body itself typically had to be removed from both the upstream and downstream flow lines in order to properly readjust the blowdown pressure and replace the valve seat. The numerous components of the pilot valve complicate the pilot valve maintenance operation, and extends the off-line pilot valve replacement time. The longer operator time required to replace safety system components, the greater the likelihood of inadvertently damaging or rupturing a flow line and the longer an operator may be exposed to a potentially hazardous environment. Accordingly, these problems have limited the acceptance of pilot controlled relief valves in a safety relief system.
The disadvantages of the prior art are overcome by the present invention, and an improved dual pilot manifold assembly for activating a safety relief valve is hereinafter disclosed. The dual pilot manifold assembly of the present invention has a relatively low manufacturing cost, may be easily installed using preassembled components standard to a particular type of relief valve, substantially reduces the likelihood of vessel pressure leaking from the safety system, and reduces the operator time and expertise required to replace a worn pilot valve in the safety relief system.